A brake unit of this kind, where the, coil spring is "loaded" by an electric motor and there is a further locking spring and a ball screw arrangement to transform the rotary motion into an axial movement, is shown and described in our EP-A-0 334 434. This electro-mechanical brake unit, in which both the brake energy and the control signal are supplied in electric form, has great advantages over both more conventional brake units, which are pneumatically supplied and controlled, and earlier attempts with electro-mechanical brake units.
However, the electro-mechanical brake unit as shown and described in EP-A-0 334 434 has the characteristic that a current supply to the control motor therein is required to obtain brake application under the action of the prestressed coil spring. This means that the brake unit does not have the safety feature with automatic brake application at the loss of the control current supply, which in this case thus is electric.
In certain instances it is a prerequisite with this feature, which in pneumatic installations often is obtained by means of a so called spring brake actuator. The feature does not only provide the necessary safety at the loss of the control current supply, but it may also give a simple and effective way of obtaining a parking brake.